Whales are sea-living mammals. They therefore breathe air, but cannot survive on land. Some species are very large indeed and the blue whale, which can exceed 30 m in length, is the largest animal to have lived on earth. Superficially, the whale looks like a fish, but there are important differences in its external structure: its tail consists of a pair of broad, flat, horizontal paddles (the tail of a fish is vertical) and it has a single nostril on top of its large, broad head. The skin is smooth and shinny, and beneath it lies a layer of fat (blubber). This is up to 30 cm in thickness and serves to converse heat and body fluids.